1. Field of the Invention
The present invention relates to a porous anode active material, a method of preparing the same, and an anode and a lithium battery employing the same. More particularly, the present invention relates to a porous anode active material comprising pores having a bimodal size distribution, a method of preparing the same, and an anode and a lithium battery employing the same.
2. Description of the Related Art
Non-aqueous electrolytic secondary batteries using lithium compounds as anodes have high voltages and energy densities, and thus have been actively studied. In particular, vigorous research has been conducted on lithium due to its high battery capacity in the early time when lithium attracted attentions as a material for anodes. However, when lithium metal is used as an anode, a large amount of lithium dendrites is formed on a surface of the lithium metal during charging, thereby decreasing charge/discharge efficiency or allowing short circuit to occur between the anode and a cathode. Further, due to instability, i.e., high reactivity of lithium, lithium anodes are sensitive to heat or an impact and have a risk of explosion. Thus, it is impossible for lithium anodes to be widely used. These problems were overcome by using carbon anodes. Carbon anodes are in so-called rocking-chair mode, in which lithium ions in an electrolytic solution participate in redox reactions while being intercalated or deintercalated between crystal planes of carbon electrodes during charging and discharging, without using lithium metal.
Carbon anodes greatly contributed to population of lithium batteries by overcoming various disadvantages of lithium metal. However, as various portable devices become smaller and lighter and have higher performance, the need for lithium secondary batteries having higher capacity is increasing. Lithium batteries containing carbon anodes essentially have low battery capacity due to a porous structure of carbon. For example, even graphite having the highest crystallinity has a theoretical capacity of about 372 mAh/g as measured in the form of LiC6, which is at most about 10% of a theoretical capacity of lithium metal of 3860 mAh/g. Thus, even though metal anodes have the above problems, many attempts have been made to increase battery capacity by introducing metals, such as lithium, into anodes.
It is known that lithium and its alloys, such as lithium-aluminum, lithium-lead, lithium-tin, and lithium-silicon, etc. may provide greater electric capacities than carboneous materials. However, when lithium or its alloys are used alone, dendrite lithium may be formed on its surface. Thus, attempts have been made to increase electric capacity while preventing short circuit by suitably mixing lithium or its alloys with carboneous materials.
However, the lithium metal materials have volumetric expansions different from carboneous materials during redox reactions and react with an electrolytic solution. When an anode material is charged, lithium ions enter the anode, thereby allowing the anode to expand and have a denser structure. Then, when the anode is discharged, lithium ions are released from the anode, thereby decreasing a volume of the anode material. In this case, due to the difference of the expansion ratio between the carboneous material and the metal material, when they shrink back, a vacant space is formed in the anode and even, electrically broken portions are generated. Thus, electrons cannot easily move in the anode and an efficiency of a battery decreases. Further, during the charging and discharging, the metal material reacts with an electrolytic solution and a lifetime of the electrolytic solution decreases, thereby decreasing a lifetime and an efficiency of the battery. To overcome the above problems due to the use of the composite material, various technique were suggested.
Japanese Laid-Open Patent Publication No. 1994-318454 discloses an anode including a mixture of metal or alloy powders in the shape of scales, carbon powders in the shape of scales, and a binder. In the anode, the metal or alloy powders are layered one upon another, parallel to a surface of the electrode, and thus when the electrode expands and shrinks during operation of the electrode, a constant pressure is applied to a whole of the electrode, thereby preventing deterioration of current collection after repetition of charge/discharge cycles. However, it is difficult to solve the above problems due to the charging and discharging only by using the mixture containing the flat powders. A stress is generated according to expansion and shrink of the metal and routes of electron transfer are greatly broken. Thus, as the number of charge/discharge cycles increases, a battery capacity greatly decreases.
Japanese Laid-Open Patent Publication No. 1997-249407 discloses an anode including highly crystalline graphite particles and fine metal particles capable of forming an alloy with lithium. Raw powders consisting of the graphite particles and the metal particles are prepared and pulverized to obtain a composite in which the crystalline graphite particles and fine metal particles are dispersed. The feature of the invention lies in the use of the fine metal particles. However, the anode is formed by simply assembling them, and thus when the metal particles expand, bonds between the metal particles and the graphite particles are broken.
Thus, there is a need for an anode active material having excellent charge/discharge characteristics without having the above problems.